The ability to measure simultaneously the size and velocity of individual particles or droplets in process flows is of considerable importance. Such measurements can be critical in evaluating particulate or smoke emissions, fuel spray combustion processes, and flow measurements in fluid systems. Prior to the invention of the laser both particle size and velocity measurements were made by physically placing a sample probe in the flow. The associated problems of flow interference and sampling errors can be eliminated using laser optical measurement techniques. The present invention concerns a laser method for simultaneous measurement of particle sizes and velocities.
The prior art in laser instruments for measuring particle velocity includes laser Doppler velocimeters, transit-time velocimeters and time-of-flight velocimeters. Laser Doppler velocimeters utilize two intersecting laser beams and are of no relevance here. The transit-time velocimeter (discussed by M. J. Rudd, in Proceedings of Second International Workshop on Laser Velocimetry, Purdue University Press, 1974, but apparently not patented) measures the time for a particle to pass across a single beam of known diameter thereby determining the particle velocity. The time-of-flight laser velocimeter (U.S. Pat. No. 3,941,477, R. Shodl, 1976) measures the time for a particle to pass between two beams of known spacing to determine the velocity. The transit-time method measures the magnitude of the velocity in the plane perpendicular to the laser beam but gives no information on the direction of particle motion. The time-of-flight method measures one particle velocity component along the interbeam axis in the plane normal to the two beams, but even this one component measurement is inaccurate if the beam spacing is not much greater than the beam diameters. The present invention includes a unique method to measure two velocity components in the perpendicular plane using a new analysis of the time-of-flight and transit-time velocimeters.
In terms of laser particle sizing instruments, a number have been invented which utilize the radiation scattered by particles passing through a laser beam for particle size measurements. Of relevance here is the nonuniform intensity distribution across a laser beam and the resulting ambiguity due to small particles passing through the center of the laser beam and large particles passing through off-center positions of lower beam intensity since both particles could scatter the same amount of laser radiation. This problem has been circumvented in the prior art by: physically confining the particles to pass only through the center of the laser beam (J. Heyder, J. Aerosol. Science, 2,341, 1971); altering the laser beam intensity distribution (U.S. Pat. No. 3,851,169, F. R. Faxvog, 1974); or utilizing the ratio of light scattered by particles in two directions to cancel the incident intensity effect (U.S. Pat. No. 3,835,315, Grawatt, 1974). This invention includes a new and unique method to eliminate the ambiguity due to variations in incident intensity.
Another concept in particle measurements is to utilize laser-induced fluorescence rather than scattered radiation for both velocity (W. H. Stevenson, et. al., Applied Physics Letters, 27,395 (1975) and R. Shodl, "Laser Two Focus Velocimetry for Use in Aero Engines," AGARD Lecture Series 90, Trenton, N.J., August 25-27, 1977) and for particle sizing by the fringe visibility method (R. dos Santos and W. H. Stevenson, Applied Physics Letters, 30,236 (1977)). This concept can significantly enhance signal to noise ratios since stray laser scattering from windows or optical components which would provide a background noise level for scattering measurements can be optically filtered out. Scattering and fluorescence can be separated because in fluorescence molecules in the particle absorb some laser energy and jump to higher energy levels. After about 10.sup.-9 sec the excited molecules drop back down to a lower energy level and emit radiation (fluorescence), but the vast majority of molecules drop to energy levels above their initial state and therefore fluoresce at wavelengths longer than the laser or exciting wavelength. Therefore, a color filter which blocks the laser wavelength can pass fluorescent radiation. This invention includes the unique concept of utilizing fluorescent radiation levels for particle volume measurements.
Relative to the prior art, the present invention provides the unique and valuable capability to simultaneously measure particle velocities and sizes. The two velocity component measurement requires a considerably less complex system than methods presently available. Also, the present system is readily adapted for three dimensional velocity measurements.